1. Field of the Invention
The invention relates to an electrochemical storage cell based on sodium and sulfur with an anode space and a cathode space which are separated from each other by a solid electrolyte and are bounded at least in some places by a metallic housing, and a fiber-like material impregnated with sulfur arranged within the cathode space.
2. Description of the Prior Art
Such electrochemical storage cells are highly suitable as energy sources. To an increasing degree they find use in the construction of storage batteries to provide the power supply of electric vehicles.
A specific example of these storage cells are those which operate on the basis of sodium and sulfur, are rechargeable and have a solid electrolyte of beta-aluminum oxide which separates the anode space from the cathode space. One advantage of these storage cells is that there are no electrochemical secondary reactions. The reason for this is that only sodium ions can get through the solid electrolyte. The current yield of such sodium/sulfur storage cells is therefore near 100 percent. In these electrochemical storage cells, the ratio of energy content to the total weight of the storage cell is very high as compared to lead storage batteries since the reactants are light and much energy is released in the electrochemical reactions.
The sulfur electrode which is formed by a fiber-like material of carbon or graphite and is impregnated with sulfur is within the cathode space. A disadvantage of these storage cells with such a sulfur electrode is that they are only moderately rechargeable since during the charging of the storage cells, liquid sulfur is formed which is deposited on the solid electrolyte and blocks further charging of the storage cell because the sulfur is nonconductive.
German Pat. No. 26 49 660 discloses a storage cell of the sodium and sulfur type, in which blocking of the electrode is prevented by a layer, arranged in the vicinity of the solid electrolyte, which layer is preferentially wetted by sodium polysulfide. The effectiveness of the layer depends on the nature of its surface. The latter is brought about by subjecting the layer to a pretreatment before it is arranged in the cathode space. However, the effectiveness of such a layer diminishes with increasing age of the storage cell. In particular, the effect of the layer is reduced by absorption of impurities from the sodium-polysulfide melt, for instance, of corrosion products and oxidation products such as are produced with advancing age in the storage cell. Surface reactions with the sodium polysulfide and the segregation of impurities from the solid electrolyte additionally reduce the surface effectiveness of such a layer. In other storage cells, the rechargeability is improved by the provision that materials which influence the properties of the sulfur in a positive manner are additionally filled into the cathode space. However, here to, there are disadvantages. Thus, for instance, storage cells, into the cathode space of which tetracyanoethylene is added, exhibit a rapid drop in capacity with increasing age, particularly when they begin to leak. A further disadvantage of such additives is that most of them are poisonous and, in addition, expensive. They cause difficulties in the recovery of failed storage cells because they cannot be separated readily from the sodium polysulfide. In addition, a corrosion-promoting effect within the storage cell is observed with some additives, particularly with arsenic fluoride and tetracyanoethylene.